Adsorption of hydrogen on zinc oxide. Energy of interaction and related mechanisms

Abstract

The heat of adsorption of hydrogen on zinc oxide (Kadox 25) has been measured at room temperature using a Calvet microcalorimeter.

Two main forms of adsorbed hydrogen have been found, coinciding with the literature types I and II. Both always occur simultaneously (although with different kinetics) and a clear-cut separation between the two forms is not achieved on the basis of their reversibility. Differential heats of adsorption decrease with coverage from 60 to 14 kJ mol^–1. Apart from the lowest coverages, where a real heterogeneity in site energies is present, the decrease is basically due to the simultaneous adsorption of type I (ca. 40 kJ mol^–1) and type II (ca. 14 kJ mol^–1), the latter prevailing at increasing coverage. The surprising feature that the ‘irreversible’ type II has a lower adsorption heat than the reversible type I is interpreted as evidence for diffusion of the adsorbate into the bulk, in agreement with kinetic observations.